Downgrade 'Unable to find purge backend' messages to info #15548

Job	Run time
test-sqlite (3.10, Django>=4.2,<4.3)	10m 23s
test-postgres (3.8, Django>=4.2,<4.3, false, --parallel)	7m 17s
test-postgres-elasticsearch7 (3.8, Django>=4.2,<4.3, false)	7m 5s
test-postgres (3.11, Django>=5.0,<5.1, false, emailuser, --parallel)	7m 34s
test-mysql (3.9, Django>=4.2,<4.3, false)	17m 14s
test-sqlite-elasticsearch8 (3.10, Django>=5.0,<5.1, emailuser)	6m 4s
test-postgres (3.12, Django>=5.0,<5.1, postgres:15, notz, false, --parallel)	10m 5s
test-mysql (3.12, Django>=5.0,<5.1, false, --parallel, mysql:8.1)	13m 31s
test-postgres (3.10, git+https://github.com/django/django.git@stable/5.0.x#egg=Django, true)	15m 8s
test-sqlite-opensearch2 (3.9, Django>=4.2,<4.3, false)	5m 15s
test-postgres (3.10, git+https://github.com/django/django.git@main#egg=Django, true, postgres:lat...	8m 18s
coverage	52s
	1h 48m 46s

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